Extruded graphite matrix containing coated particles



M. J. SMITH Dec. 31, 1968 EXTRUDED GRAPHITE MATRIX CONTAINING COATEDPARTICLES Original Filed Aug. 23, 1963 FIG;

FIG. 6

INVENTOR MA R K J. S M TH United States Patent 3,419,468 EXTRUDEDGRAPHITE MATRIX CONTAINING COATED PARTICLES Mark J. Smith, Wilson, N.Y.,assignor to Air Reduction Company, Incorporated, New York, N.Y., acorporation of New York Continuation of application Ser. No. 539,836,Apr. 4, 1966, which is a division of application Ser. No. 304,194, Aug.23, 1963. This application July 6, 1967, Ser. No. 653,300

1 Claim. (Cl. 176-69) ABSTRACT OF THE DISCLOSURE A nuclear fuel rod ofheterogeneous composition and predetermined length consisting of nuclearfuel particles with unbroken coatings distributed throughout an extrudedgraphite matrix.

This application is a continuation of a prior co-pending application,Ser. No. 539,836 filed Apr. 4, 1966, and now abandoned, whichapplication is a division of prior copending Application Ser. No.304,194, filed Aug. 23, 1963, now Patent No. 3,255,277.

This invention relates to a method and apparatus for extrusion of agraphite matrix containing coated nuclear fuel particles, and moreparticularly the avoidance of broken coatings during fabrication and inthe finished article.

Rod-like articles made by an extrusion process are normally extrudedcontinuously in long lengths and are then cut to the desired finishedlength. Fuel rods containing coated fuel particles in a graphite matrixcan be formed by extrusion methods but cannot be extruded continuouslyand cut to length inasmuch as any cutting operation would necessarilydestroy or break the coating on the fuel particles which form part ofthe severed surface. The coating commonly comprises pyrocarbon,pyrographite or the oxides of metals such as aluminum and beryllium andis able safely to retain gases which may form within the coating whenthe coated particle is iradiated. If the coating is broken or destroyed,these gases can escape and it is necessary that such escape beprevented. The fuel particles primarily contemplated are nuclear fuelparticles, usually composed of uranium carbide, uranium oxide, thoriumcarbide, thorium oxide, or alloys thereof. The gasses that are formedunder irradiation when these materials react with neutrons to undergonuclear fission are termed fission product gases and may be comprised ofisotopes such as krypton85 and xenon-133.

In accordance with the invention, rupture or destruction of the particlecoatings is avoided by taking advantage of this fact. It has been foundthat when the material of an extruded rod is broken while still hot andin a pliable or plastic condition, the rupture takes place in the bindermaterial surrounding the coated particles, selectively avoidingrupturing the particle coatings. This preferential rupturing has beenfound to be characteristic of a hot carbonaceous extrusion containing anadmixture of the coated particles. The length of rod broken off whilehot is made slightly greater than the desired finished length and therod is then compressed and formed with square ends as desired while thematerial is still hot. The finished rods made in this way are found tocontain no fuel particles that have broken coatings.

Further in accordance with the invention, apparatus is provided tofacilitate the carrying out of the above described method.

Other features, objects and advantages will appear from the followingmore detailed description of an illustrative embodiment of theinvention, which will now be given in conjunction with the accompanyingdrawings.

In the drawings,

FIG. 1 is an axial or longitudinal sectional view of an illustrativeembodiment of apparatus useful in practicing the invention;

FIG. 2 is a perspective view of a split die like that shown in axialsection in FIG. 1;

FIG. 3 is a perspective view of a die shell for supporting the split dieshown in FIGS. 1 and 2; together with a retaining nut therefor;

FIG. 4 is a perspective view of a die adapter or nozzle for coupling thedie shell and split die to a source of molten material to be extrudedinto the die;

FIG. 5 is a view, mostly in axial section, showing an extruded charge ofplastic material in the split die after the die has been detached fromthe nozzle and before final molding of the plastic material, the diebeing shown as between press jaws with molding caps at each end of thedie ready to form flat ends upon the extruded charge and mold it to adesired finished length; and

FIG. 6 is an elevational view of a finished rod after removal from thedie.

Referring to the drawings, FIG. 1 shows in axial section illustrativeapparatus for molding nuclear fuel rods in accordance with theinvention. A die adapter or nozzle 20 is shown which will be attached inknown manner to the container of a hydraulic ram. The container willhold the mix which is to be extruded through the nozzle 20 to form thenuclear fuel rod.

A suitable mix that has been successfully employed in practicing theinvention contains five percent by weight of coated nuclear fuelparticles in a matrix which is composed of about two-thirds by weight of200 mesh graphite and one-third #30 coal-tar pitch. In this matrix thefuel particles constitute about 0.10 to 0.15 gram per cubic centimenterof the matrix. The mix has a softening point of about C. For extrusion,the mix is maintained in the range between and 125 0., being preferablyat about 120 C. in the container.

The mix is extruded into a split die to form a cylindrical rod. The diehalves 22 and 24 are shown in axial section in FIG. 1. The die halvesmay be doweled or bolted together in any known manner and may be groovedat the parting line to facilitate prying the halves apart for removal ofthe finished rod from the die. During extrusion and molding, the die iscontained within a shell 26 and retained therein by set screws 28 and 30which engage in grooves in the die halves as shown. The parting linebetween the die halves is indicated at 32. The combination of the dieshell 26 and the enclosed die halves is coupled to the nozzle 20 bymeans of a retaining nut 34, which may include a steel bearing ring 36tack welded to the main body of the nut. The nut 34 makes a threadedengagement with the outer surface of the nozzle 20 as shown.

The split die halves are shown separately, in isometric view in FIG. 2,the die shell with retaining nut is shown in FIG. 3, and the nozzle isshown in FIG. 4.

In the molding operation, with the nozzle 20 coupled to the die by meansof the die shell 26 and retaining nut 34 as shown in FIG. 1, the mixfrom the container is extruded by means of the hydraulic ram until theforward boundary of the extrusion reaches a point in the die aboutone-sixteenth of an inch short of the open end of the die. At this timethe temperature of the die is about C. and the extruded material in thedie is in a readily rupturable plastic state, and sufiiciently viscousto substantially retain its shape in the die.

Next, the retaining nut 34 is uncoupled from the nozzle and removed fromthe die shell. With the die still hot, the die and die shell areseparated as a unit from the nozzle, thereby rupturing the extruded massat the nozzle end by preferential rupture in the matrix material withoutdanger of breaking or destroying the coating of any fuel particle whichmay be included along the surface of cleavage. The separating motion isusually accompanied by a slight downward motion on the outer end of thedie, producing a bending force to insure the fracture transverselythrough the matrix material.

It will be noted that, as is ordinarily the case, the moldingtemperature of the matrix material is not high enough to have anydeleterious effect upon the fuel particles.

FIG. 5 shows the extruded mass which is left in the die after theseparation has been effected. At the right hand end of the die as viewedin FIG. 5, the somewhat irregular shaped leading edge 38 of theextrusion is seen just inside the end of the die opening. At theleft-hand end of the die, the also somewhat irregularly broken surface40 of the extrusion is seen just inside the left-hand end of the dieopening proper.

With the die still hot, the assemblage of die shell 26 and die with theextrusion therein is placed between press jaws 42, 44 of a suitableconventional press, with shouldered molding caps 46 and 48 at theopposite ends of the die and the plungers on the molding caps are forcedinto the die at the two ends until the shoulders of the respectivemolding caps are stopped at the end surfaces of the die. By thisoperation, flat ends are formed on the fuel rod while the extrusion isstill in a plastic state and the length of the finished rod isdetermined precisely by the distance between the confronting projectionsof the stud portions of the two molding caps. The tolerances of therough, or broken off, length of the fuel rod are fairly closely heldaccording to the present breaking-off step and, in 12 inch articles forexample of the type generally contemplated, the maximum foreshorteningaccomplished during the sizing operation from the opposite ends isapproximately Ms" total. No absolute limit has been found for theincrease in compaction density of the extruded rod in its confineswithin the enclosing split die, however, and at least ideally thematerial may be forced to reach or approach its theoretical density whencompressed from the opposite ends. In practice, of course, the rod iscompressed by the molding cap studs only to the point where its lengthis reduced to the length desired, and no farther.

After molding and cooling, the set screws 28, 30 is retracted, the dieremoved from the die shell, and the finished rod is then removed byprying apart the two halves of the split die, whereupon the rod appearsas shown in elevation in FIG. 6.

While illustrative forms of apparatus and methods in accordance with theinvention have been described and shown herein, it will be understoodthat numerous changes may be made without departing from the generalprinciples and scope of the invention.

I claim:

1. A nuclear fuel rod of heterogeneous composition comprising anextruded length of graphite containing a distribution therein of coatednuclear fuel particles, said extruded length having end portionscompacted toward the center of said length, the population of saidparticles being characterized by the essential absence therein ofmembers exhibiting broken coatings resulting from application ofshearing forces transverse to said length.

References Cited UNITED STATES PATENTS 2,593,507 4/1952 Wainer 264-.53,121,047 2/1964 Stoughton et a1. 17691 3,124,625 3/1964 Sheinberg etal. 264.5 3,135,665 6/1964 Koutz et a1. 176-91 3,173,973 3/1965 Brockway264.5 3,224,944 12/ 1965 Turner et a1 264.5 3,230,280 1/1966 Kennedy264.5 3,293,332 12/1966 Ingleby 264.5

CARL D. QUARFORTH, Primary Examiner.

R. L. Grudziecki, Assistant Examiner.

US. Cl. X.R. 17667; 264.5

